/Engine/Source/Runtime/AudioMixerCore/Public/AudioMixerNullDevice.h
FMixerNullCallbackThis class, when started, spawns a new high priority thread that exists to query an FAudioMixerPlatformInterface and immediately throw out whatever buffers it receives.
class FMixerNullCallback : protected [FRunnable](API\Runtime\Core\HAL\FRunnable)